The invention relates in general to the field of vibration cancellation and in particular to a compact assembly for cancellation of vibration in structural members.
For many years, effort has been directed toward the design of an apparatus for attenuating or canceling vibration in structural members. Typical vibration attenuation devices employ various combinations of the mechanical system elements to adjust the frequency response characteristics of the overall system to achieve acceptable levels of vibration in the structures of interest in the system. One field in which these isolators find a great deal of use is in aircraft, wherein vibration cancellation systems are utilized to cancel mechanical vibrations which are associated with the propulsion system and which are generated by the engine, transmission, propellers, rotors, or proprotors of the aircraft.
Minimization of the length, weight and overall size of the isolation device is an important consideration in the design of an aircraft vibration cancellation system. This minimization is particularly important in the design and manufacture of helicopters, which are required to hover against the dead weight of the craft and which are in many ways more constrained in their payload than fixed wing aircraft.
A marked improvement in the field of vibration cancellation was disclosed in U.S. Pat. No. 989,958 entitled xe2x80x9cDevice for Damping Vibrations of Bodiesxe2x80x9d issued Apr. 18, 1911 to Hermann Frahm, and which is incorporated herein by reference. This patent discloses a vibration cancellation device in which an xe2x80x9cauxiliary bodyxe2x80x9d is arranged within or on the main body to counterbalance and cancel oscillating forces transmitted through the device. This device employs the principle that the acceleration of an oscillating mass is 180 degrees out of phase with its displacement to cancel the undesirable motion.
Frahm recognized that the inertial characteristics of a sprung mass could harness out-of-phase acceleration of the auxiliary body to generate counter-balancing forces to attenuate or cancel vibration in a structural member.
Within the Frahm device, the auxiliary body is excited by the vibration of the structural member to which it is attached. The auxiliary body and connecting spring in the Frahm device form a classical spring and mass assembly, the natural frequency, xcfx89n, of which is given by:
xcfx89n={square root over (k/m)}
Where: k=the rate of the connecting spring
m=the mass of the auxiliary body
The action of the Frahm device depends upon the principle that the auxiliary body will resonate at its natural frequency xcfx89n. An attempt to excite the Frahm device by application of a vibration at the device""s resonant frequency xcfx89n will impart high-amplitude excitation to the auxiliary body. This high-amplitude excitation of the auxiliary body will, in turn, exert a series of counter-impulses to the fixed end of the connecting spring 180 degrees out-of-phase to the vibration imparted to the Frahm device. This counter-vibration has the effect of substantially or completely canceling the imparted vibration at the resonant frequency of the device.
Although the Frahm device was a significant improvement in the art of vibration cancellation, there remains in the field a continuing demand for improvements allowing for a reduction of the size of such devices without sacrificing the ability to attenuate or cancel vibration.
The present invention disclosed herein comprises an improved vibration cancellation device designed to overcome many of the shortcomings inherent in prior devices. In many embodiments, the vibration cancellation device is smaller in scale than prior designs, facilitating more versatility with respect to design options.
One embodiment of the present invention is a vibration cancellation device incorporating an auxiliary mass having a center, a principal axis of motion, and a central plane passing through the center of mass and orthogonal to the axis of motion. This embodiment incorporates a first set of substantially planar surfaces substantially orthogonal to the axis of motion and facing in a first direction toward the central plane, as well as a second set of substantially planar surfaces substantially orthogonal to the axis of motion and facing in a second direction substantially opposite the first direction. The auxiliary mass is suspended by a first set of springs acting on the mass through the first set of surfaces and a second set of springs acting on the mass through the first set of surfaces.
A second embodiment of the present invention is a vibration cancellation device incorporating a mass having a center, an axis of motion, and a central plane passing through the center of mass and orthogonal to the axis of motion. The auxiliary mass incorporates a first set of mounting features disposed on a first set of surfaces facing in a first direction toward the central plane of the mass and a second set of mounting features disposed on a second set of surfaces facing in a second direction substantially opposite the first direction. The mass is suspended by a first set of springs acting on the mass through the first set of mounting features and a second set of springs acting on the mass through the second set of mounting features.
A third embodiment of the present invention is a vibration cancellation device comprising an auxiliary mass having a center, an axis of motion, and a first central plane passing through the center of mass and orthogonal to the axis of motion. Additionally, the auxiliary mass has a second central plane passing through the center of mass and orthogonal to the first central plane and having a first and second side and a third central plane passing through the center of mass and orthogonal to the first and second central planes and having a first and second side.
This third embodiment of the present invention uses a set of four springs to suspend the auxiliary mass, with one spring being disposed in each of the four quadrants, and the springs disposed in opposite diagonal quadrants acting together in opposition to the remaining two springs.
A fourth embodiment of the present invention is a vibration cancellation device incorporating a frame having a first and second set of spring pockets. The pockets in the first set are shaped and disposed to accept and retain a first set of springs along a first set of spring axes parallel to the axis of motion. The pockets in the second set face opposite the first set of spring pockets and are shaped and disposed to accept and retain a second set of springs along a second set of spring axes parallel to the axis of motion.
The frame is suspended by a first set of springs disposed within the first set of spring pockets and a second set of springs disposed within the second set of spring pockets. The relative positions of the pockets are defined and limited in that the first set of spring axes and the second set of spring axes are not coincident.